A flowmeter suitable for measuring the rate of supply of fuel to a petrol engine or the rate of supply of spraying liquid to the boom of agricultural or horticultural spraying equipment includes a bladed rotor element which is rotated by liquid passing through the meter, the speed of rotation being proportional to the rate of flow. Such flowmeters may be required to operate over a range of flow rates from 0.25 to 30 gallons per hour. The lower end of the range dictates that the maximum diameter of the bore in which the rotor element is mounted should be generally less than 0.25 inches.
At low flow rates there is an unacceptable drag effect if an electromagnetic device is used for sensing the speed of rotation of the rotor element. Although an inductive sensor overcomes this problem, the element must be made of metal, or must include a metallic insert. It is then costly to manufacture the element from a metal which is corrosion resistant and the element is heavy, which means higher bearing friction and hence poor flow rate characteristics. It is preferable therefore to make the rotor element of a plastics material, which offers the advantage of low cost, low weight, low production spread in terms of dimensional accuracy, and a choice of materials which have suitable chemical and physical properties.
To detect the speed of rotation of the rotor element an optical sensing device has been proposed as an alternative to an inductive or electromagnetic device. In the optical device a beam of light is transmitted across the bore so that it is interrupted by the rotating blades of the rotor element. A photodetector to which the beam is applied generates an electrical signal of frequency representing the frequency at which the beam is interrupted, and hence the speed at which the element is rotated. For satisfactory operation, this optical device relies upon a high degree of transparency of the liquid passing through the meter. Further, spurious signals are produced if the liquid includes gas bubbles which interrupt the light beam. It is found that there may be errors of up to 25%, particularly if the "lens effect" of liquid within the bore is used to focus the light beam on to the photodetector.